The encoding and interpretation of FGF signals in mammalian cell fate choice

在胚胎发育过程中，细胞变得彼此不同，并且在时间和空间上受到高度调节。这种多样化的过程受到细胞内转录因子的协调活动的调节，这些转录因子决定细胞成为什么样的细胞，并受到细胞发出和接收的信号的调节，这些信号使它们能够相互交流状态。转录因子作为细胞身份的主人，其工作方式越来越被人们所理解：它们与DNA结合，可以激活或抑制基因组的表达，这些基因组将赋予细胞特定的功能和身份。转录因子以组合方式起作用，即一些基因表达程序是叠加几个单独转录因子的活性的结果。信号被特定的受体接收，并通过细胞中的信号转导途径传递给它们的靶点，并用于共享有关它们状态的信息。信号如何与转录因子的活性相互作用，仍然有待讨论。这可能是信号通路的效应子像转录因子一样起作用，即它有助于某种编码，为每个细胞定义独特的身份。或者，效应子可能具有更允许的作用，例如通过打开转录因子作用的分子景观（染色质结构），或作为在单细胞水平上增强或减弱那些转录因子活性的因子。我们的建议旨在区分这些可能性集中在FGF/MAPK信号通路，在两个实验系统，使自己的定量单细胞分析：植入前小鼠胚胎及其相关的，ES cells.FGF是一种细胞外信号分子，触发信号通路，激活蛋白激酶，MAPK。已知MAPK的活性影响细胞的行为，特别是它们分化成更特化的细胞类型。FGF/MAPK活性在发育中的胚胎中广泛存在，尽管许多不同的细胞都接受这种活性，但不同细胞和不同时间的反应是不同的。我们的项目旨在了解这种观察的基础。通过MAPK的信号传导在正常和病理条件下非常常见，因此我们确信我们的发现将产生超出我们实验系统的影响。为了更好地了解FGF/MAPK信号传导的作用模式，我们将专注于哺乳动物胚胎中最早的细胞命运决定。在胚胎植入子宫之前，一组幼稚细胞，内细胞团（ICM），被细分为两个不同的群体：原始内胚层（PrE），将产生胚外组织，以及幼稚多能外胚层（Epi），将产生胚胎。成为PrE的决定取决于少数转录因子和FGF/MAPK信号传导。我们可以使用工程化的胚胎干细胞（ES）在培养中重现这些事件，其中我们可以通过激活转录因子诱导命运决定事件，并通过操纵FGF/MAPK信号转导调节反应。通过专注于单细胞，我们可以了解决策的细节，这不能从粗糙的approaches.An问题的一个重要组成部分是开发报告MAPK信号在单细胞和链接信号活动的转录输入和输出的过程。这种结合将使我们能够获得该过程的定量动态数据，并为细胞决策提供新的数学模型。

During the development of an embryo, cells become different from each other and do so in a manner that is highly regulated in time and in space. This process of diversification is regulated by the coordinated activity of transcription factors inside cells, which determine what the cells become, and by signals emitted and received by cells that allow them to communicate their states with each other. The workings of transcription factors as the masters of cell identity are increasingly well understood: they bind to the DNA and can activate or repress the expression of cohorts of genes that will endow cells with specific functions and identities. Transcription factors act in a combinatorial manner, i.e. some gene expression programs are the result of overlaying the activities of several individual transcription factors. Signals are received by specific receptors and passed on to their targets via a signal transduction pathway in the cell and are used to share information about their state. How signals interact with the activity of transcription factors, remains open to discussion. It could be that an effector of a signalling pathway acts like a transcription factor i.e. that it contributes to some sort of code that defines a unique identity for every cell. Alternatively the effector might have a more permissive role e.g. by opening up a molecular landscape (chromatin structure) for the action of the transcription factors, or as a factor that enhances or weakens the activity of those transcription factors at the level of single cells. Our proposal aims to distinguish amidst these possibilities by focusing on the FGF/MAPK signalling pathway, in two experimental systems that lend themselves to quantitative single cell analysis: the preimplantation mouse embryo and its correlate, the ES cells.FGF is an extracellular signalling molecule which triggers a signalling pathway that activates a protein kinase, MAPK. The activity of MAPK is known to influence the behaviour of cells, in particular their differentiation into more specialized cell types. FGF/MAPK activity is widespread in developing embryos, and although many different cells receive this activity, the response is different in different cells and at different times. Our project is aimed at understanding the basis for this observation. Signalling through MAPK is very common in normal and pathological conditions and we are therefore certain that our findings will have an impact beyond our experimental system. To gain a better understanding of the modes of action of FGF/MAPK signalling, we will focus on the earliest cell fate decisions in a mammalian embryo. Before the embryo implants in the uterus, a group of naïve cells, the Inner Cell Mass (ICM), becomes subdivided into two different populations: the Primitive Endoderm (PrE), that will give rise to extraembryonic tissue, and the naïve pluripotent Epiblast (Epi) that will give rise to the embryo. The decision to become PrE depends on a small number of transcription factors and FGF/MAPK signalling. We can reproduce these events in culture using engineered Embryonic Stem (ES) cells in which we can induce the fate decision event by activating a transcription factor and modulate the response by manipulating FGF/MAPK signalling. By focusing on single cells we can learn details of the decision which cannot be gauged from coarser approaches.An essential part of the problem is to develop reporters for MAPK signalling in single cells and link signalling activity to the transcriptional inputs and outputs of the process. This combination will allow us to obtain quantitative, dynamic data of the process, and inform novel mathematical models of cell decision making.